jargon.artrocity.com
New Hacker's Dictionary the Jargon File v 4.4.6
1. In a regular expression or pattern match, the text which was matched within grouping parentheses
2. The part of the pattern which refers back to the matched text.
3. By extension, anything which refers back to something which has been seen or discussed before. “When you said ‘she’ just now, who were you backreferencing?”